1. Field of the Invention
The present invention relates antennae comprising device for exciting waveguides in the circular mode.
2. Description of the Prior Art
In order to excite a circular waveguide from a microwave line, it is necessary to change the mode of propagation of the wave transmitted by the line.
In fact, in microwave lines which are in current use such as coaxial lines, two-wire, three-plate or microstrip lines, the mode of propagation of waves is a transverse electromagnetic mode (TEM mode).
The mode of propagation of waves within a waveguide is a transverse electric (TE) mode or a transverse magnetic (TM) mode.
The preferential mode of excitation of a circular waveguide is the circular mode (TE 11 or TM 11).
In order to change from a TEM mode to a mode guided in circular polarization within a circular waveguide, two solutions are already known.
In a first solution, the initial step consists in carrying out an electric coupling. This coupling permits a transition from the TEM mode to the TE 10 mode in a rectangular waveguide. The second step consists in carrying out a coupling by transition in order to change-over to the TE 11 (rectilinear) mode in a circular waveguide. It is then necessary to change-over from the TE 11 mode to a circular mode. This coupling operation is usually performed by a polarization rotator of the iris type or dielectric-plate type.
The second solution consists in coupling the circular waveguide by means of two probes disposed at right angles. The probes are fed by waves of equal amplitude having a phase shift of .pi./2 and transmitted by microwave line. The phase shift can be carried out prior to feeding of the probes, in which case said probes are located in the same plane. The phase shift within the waveguide can take place by relative displacement of the probes by a wavelength equal to (.lambda.g)/4 where .lambda.g is the guided wave length.
The two known solutions are usually complex and the excitation devices obtained are bulky, particularly in the case of the first solution.
In both cases of the second solution, the polarization rotator must be fed by two channels having the same power. It is therefore necessary to make use of a power divider which is capable of producing an equitable energy distribution in each channel.
In the first case of the second solution, a phase-shifter is usually adopted for the purpose of phase-shifting the probes which feed the waveguide.
Apart from the disadvantages which arise from complexity and bulk, a third additional disadvantage arises from the passband of the device since it is usually narrow and consequently ill-suited for many applications which require a very large bandwidth. One known solution, however, permits an increase in width of the passband by making use of a so-called "orthogonal double ridge" waveguide. This waveguide is machined so as to have longitudinal recessed portions which result in a waveguide having a channeled cross-section. The manufacture of waveguides of this type is clearly more complex than in the case of ordinary waveguides and consequently entails higher capital cost.